The investigators' long-term objective is to make possible primary prevention of non-Hodgkin's lymphoma (NHL), an increasingly common and often fatal cancer with an etiology that is incompletely known. The specific aim is to investigate, through a broad interdisciplinary collaboration, the etiologic role of factors that entail immune modulation. The study will be large enough to allow analyses of these factors separately for several distinct subgroups, such as B-cell NHL, T-cell NHL, nodal/extranodal disease and chronic lymphocytic leukemia (CLL). Conditions and exposures to be characterized in detail include: Type-1 allergy, pattern and postponement of infection during childhood, chronic infections, autoimmune diseases, UV-light exposure, vaccinations, drugs that affect the immune system, and - notably for T-cell lymphomas - exposure to EBV and other viruses. Drawing on experience from nationwide collaborative studies between the major research centers in Denmark and Sweden, they propose to conduct a large, nationwide, population-based case-control study. A separate network for rapid case ascertainment will be set up to identify all individuals aged 15-74 years with a newly diagnosed, morphologically confirmed, and histopathologically uniformly classified NHL or CLL. Population controls, frequency-matched to the cases by sex, age, and country, will be regularly selected from population registers. Following informed consent, a blood sample will be obtained from all participants and a telephone interview will be carried out by means of a structured questionnaire. Self-reported information will be extensively completed and validated through linkages to national databases. They anticipate an 80 percent or higher participation rate. During a two-year period (two and a half years for T-cell NHL), they expect to include at least 2,000 patients with B-cell NHL, 400 patients with T-cell NHL, 525 patients with CLL, and approximately 3,000 population controls. The investigators state that the large size enables them to detect, with 80% power at a 5% level, factors that increase NHL by a factor of 1.3 or more for B-cell NHLs, 1.5 or more for T-cell NHLs, and 1.5 or more for CLL at an exposure prevalence of the risk factor in the general population of 1.5%. Major subgroups within B-cell NHL can be analyzed with similar statistical precision. The investigators state that the efficiency of the existing infrastructure, the ready access to nationwide population-based databases, the extensive previous experience with high participation rates, willingness to provide blood samples, and ready access to histopathological specimens provide a unique and effective platform in Scandinavia for such a large and comprehensive study.